Pretty much forever. While there were debates about the seat of intellect (most famously between the Aristotelians and the Platonists), those were almost purely philosophical rather than scientific in nature.

The earliest preserved discussion of the function of the brain is in the Edwin Smith papyrus, from ca 1600 BC, but it's believed to be a copy of a much earlier text from ca 3000 BC. It's quite accurate in its discussion of gray matter.

But basically, understanding trauma is one of the earliest arenas of medicine. Trauma in war was common, obviously. It is easy to see that people with trauma to their limbs or parts of their torso can talk about their wounds. But the guy that survives head trauma all of a sudden can't speak, seems oblivious to his surroundings, or has lost some of his senses or motor skills.

So yeah, head trauma teaches a lot about the function of various parts of the brain. Tough way to learn though.